A gram of fat has 9 calories, so you would multiple the fat grams by 9. That number would have to be 30% or less of the total calories. So in your example, the 2.6 X 9 = 23.4, which is 23.4% of 100 calories - well under the 30%.
That's the way I usually calculate - if anyone knows another way or an easier way, do share!